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MG TD TF 1500 - Speedometer correction for 5 Speed gear box

Dear all,
some years ago I installed a 5 gear FORD gearbox on my '53 TD. I use the kit from Hi-Gearengineering and it works very well without any Major Problems.

Anyway, the Speedometer indicate everything but not the real Speed. I used the cable which came with the kit but the Ratio on the 5 gear box is different to the original box. This was well known and not I surprise for me. Even the Installation instruction which comes with the Hi-Gearengineering kit mentioned that issue and they recommend making a speedometer adjustment or using a mini-gearbox to fix the Problem.

Over the last 3 years this issue was not so important for me and so I accept the wrong Speed indication. Now since last week the Speedo Cable is broken and I have to think about a final solution. Due to the fact, that I have my own small machine shop with all I need to make such a mini-gearbox as it is available from Speedograph-Richfield Ltd.

Long story but now my question for you:if
Is there anybody who knows the ratio required for such a mini-gearbox? Or if you have such a mini-gearbox can you check out what ratio you have? And please let me know which ratio your rear axle has.
By the way, my rear axle ratio is 4.3*1
Thank you,
Guenter


Guenter

Guenter - Speedograph Richfield in the UK is the best source. Give them your true (GPS) vehicle speed and the speedometer reading, and they will make a converter to correct, that will fit between your transmission and cable. This will take into account any drivetrain alteration.

Tom Lange
MGT Repair
t lange

Thank you Tom,
yes I understand your point, but the challenge for me is to do it myself.
I do not have any speedo reading so far, as I said before, the cable is broken.
So what I need is the ratio of any other mini-gearbox and the Information of the rear axle Ratio. If I have this Information I can build the first prototype. With this Mini-gearbox installed I can follow your recommendation to compare the reading with a GPS "true" vehicle Speed. At that Point I have to make the decision to make any correction on the gear wheel set or not.
Cheers,
Guenter
Guenter

I understand the desire to do it yourself and wish I had the talent/skills. Perhaps if youo can recall the amount of error that was there before the cable broke you can make a pretty good estimate. For instance if you thought you were going 50 mph but the speedo said 40 mph you have a 5:4 ratio so if the number of teeth on the driven gear are 50, the the number on the output gear should be 62+. I think 60 teeth to 75 teeth would also work. I guess you could then produce a set of geers plus or minus and sneak up on the correct one (and have some spares in case you change the size of the tires or change the rearend ratio). Of course, I have no idera how hard it is to make gear wheels, how hard it is to get to the mini-gearbox or how hard it is to swap out the gears. Nonetheless, I think it's amazing to even try it. Good luck.

Jud
J K Chapin

Guenter, can you see the end of the broken cable? There are formulas for doing this conversion. One of the inputs is the number of revolutions of your speedometer cable in top gear for a known distance. This done by pushing the car for a known distance, or number of revolutions of a tire. I'll have to dig back into my archives to find the details. Bud
Bud Krueger

Hi Gunter,

Here is the information for re calibration of your speedo perhaps you can work out the ratio of the gearbox from these figures.

If not you can get John to recalibrate your speedo for you he did mine and it is spot on .

Heres the link

http://www.jdo1.com/speedo_calibration_1.html

Cheers Chris
C A Pick

Jud,
Thank you for your comment. It is not so difficult to make a gear wheel, if you have the equipment. It takes less than one hour to do it.

@But, thank you But, I understand your Point. Unfortunately, if I pull out the cable it will be disconnected from the gear box. I can use another cable to do that but I have to find one, which will fit into the gear and so one.
It is much easier if the Information gear Ration of such a mini-gearbox is available. Then I can do a prototype and in the second Loop I can do any corrections.
Guenter
Guenter

Guenter, look at the face of your speedometer. You should see a number, perhaps 1600. This is the number of speedometer cable rotations per mile. Below is an image of mine. This is 16 revolutions per 1/100 of a mile. If you can move your car 52.8 feet and count the number of turns of your cable for that distance (1/100 mile) you can compute the gear ratio for your correction box. It's the ratio of your number turns to 16. Bud

Bud Krueger

Guenter, I understand your problem and your wish to make a gearbox to correct the speedo. What I would do is obtain'a machinists tachometer to measure the rpm of the speedometer output on the gearbox. Jack up the rear wheels, run the engine and compare the engine rpm to the rpm shown on the tacometer that is measuring the speedo drive output. This of course would be done in 4th gear or whatever gear has a 1:1 input:output.
Cheers, Hugh Pite
H.D. Pite

Hello Guenter. When I make up the conversion boxes here in the US, I use a Stewart-Warner 666 drive joint. I have to make or modify 6 parts to fit the box. It attaches to the gearbox and the speedo cable attaches to it. For conversion of the 5.125 to the 4.3 gears the 1.2020 ratio is the closest. I've attached a couple of pictures. the first gives the size the second shows the gear box open.

R Taras

This is the second picture.

Butch

R Taras

I am also testing home built gearboxes on this side of the pond for the 4.3 CWP conversion but the ones I make are designed to run on the back of the speedo. Has been tested in the lathe for several hours and runs fine. Here's a few photos of the prototype.

Regards
Declan
Photo 1

Declan Burns

Photo 2

Declan Burns

Butch and Declan,
this is exact what I want. Can you please tell me what ratio you have selected and if your car has a FORD 5-Speed box or the still the old MG Box? What ratio has your rear axle to made my own gearbox: Still the original MG-TD axle 8/41 (5.125:1) or any other, maybe the MGA box with 10/43 (4.3:1)?
If I have this information I can do my own calculations and I can design my own gear box.
The benefit with a gearbox solution is than not only the Speedometer works correct but also the distance measuring. I know I can buy such a little gearbox somewhere but I like to make it myself. That’s the challenge for me. In the Picture you see a small part of my Workshop.
Cheers,
Guenter


Guenter

Guenter,
My car still has the original TD gearbox. I'm in the process of changing the rear end to the 4.3 MGA CWP.
I chose the step up ratio of 1.2 :1. I don't know what ratio would be required for the Ford gearbox but I'm sure someone will know. The input shaft on my step-up gearbox could easily be adapted to take the Ford cable. It has the correct thread to go on the speedo (M12x1mm). If you Need any assistance I will gladly help you. I have workshop drawings.
Regards
Declan
Declan Burns

Thank you Decan for your help I’ll see if somebody from the community will have information on the 5-gear FORD Box. I like your idea to install the mini-gearbox behind the dash direct on the instrument. It is not to complicated to made such a gear box. With a ratio of 1.2:1 I need a gear with 12 teeth and one with 10 teeth. In addition I need one gear in between to give the output the right turning direction.
As an alternative crossed helical gears can be used, which make the gearbox smaller and more simple. The gears can be made on the lathe a milling machine is in that case not needed.
OK, I should wait for information’s with the 5-Gear FORD box after that I will make decisions.
Any information is appreciated.
Thank you
Guenter


Guenter

Guenter,

The Hi-Gear website gives transmission ratios for both the TD/TF and the Sierra gearbox well as speeds for the following ratios with the 5-speed: (http://hi-gearengineering.co.uk/content/view/13/28/)

I'm mathematically challenged but maybe someone who's not can can do the conversion for you.

Gene
Gene Gillam

Gene,
thank you for your help, but what you found is the ratio of the drive train from the Engine to the rear wheels. This is for sure very important as well but it doesn’t answer my question.
What I need is the ratio from the drive train to the Speedometer. On the main Gear box there is a Speedometer connection pinion where the speedo cable is attached to. Unfortunately the ratio is different on the original MG-gearbox to the FORD-gearbox. To find out the ratio you typically have to follow the instruction coming from the Speedometer calibration companies, like Speedograph in Richfield, UK. Any other company will ask the same question and they propose a method how to get this information. Typically you have to place a cardboard arrow at the end of the protruding inner speedo cable, mark the wheel with chalk at the bottom of the measured wheel. Push the car forward as described in the exercise instruction. This procedure delivered the required information to calculate the mini-gearbox ratio.
Unfortunately, I cannot do this exercise, because my speedo cable is broken.
Anyway, If somebody has done this exercise with an MG-TD, which is converted with a FORD gearbox and a MGB rear axle with a 4.3:1 ratio I can use this information for my calculation and I can make a prototype as described before.
The information’s getting from Butch and from Declan is helpful but is not all I need. The 1.2:1 or the 1.2020: is valid for the original MG-gearbox but not for the FORD-gearbox.
Guenter
Guenter

Hello Guenter. When the differential conversion was figured out by Carl Cederstrand(he is a Phd engineer), he researched the torque carrying capacity of the speedo cable. He said that putting a auxillary gearbox behind the speedo would cause excessive stress on the cable as it was not designed to carry that much load and would break. That is why he researched and decided on using a gear box driven off the transmission.
You need to find out what the gear ratio is for the speedo drive on the transmission that you have. It should be in the literature of the car that the trans came from. Or you can find what speedo was used in the car and ask a speedo shop what the turns per mile is for that speedo. They should have it in a reference book.

Butch
R Taras

Guenter,
I don't think using 12 and 10 teeth is a good idea. That will not run smoothly. I use 2x30 and 1x25 teeth Module 0.5 and you need to add approx 0.2 to 0.25mm extra between shaft centres to get the correct backlash. If you don't do this you could run into the problems that Butch has correctly mentioned. It must run smoothly. I have experimented a lot with this and the box runs like a swiss watch so I don't anticipate any cable issues. I run it in in the lathe. I am also building a box with a bevel stage that connects on the TD gearbox and comes out at 90° towards the front of the car to reduce the cable bending. You can buy the spur gears from Conrad Electronic or Mädler. VW beetles had a reversing light after market accessory that ran off the back of the speedo made by Hella.
Regards
Declan
Declan Burns

Guenter; Try this link for information on the T9 gearbox and ratios.

www.the-wizardsden.com/html/type_9_secrets.html
C.R. Tyrell

Guenter,

I have a Hi-Gear 5 speed and just went out to my car, disconnected the speedometer cable from the speedometer, made a "degree wheel" and a pointer and got absolutely no where. There is so much slop in the cable that sometimes one rotation or the driveshaft will give you 67.5 degrees and the next turn will give you 127 degrees. You can watch the pointer continue to move after the driveshaft is fully stopped - it doesn't seem to matter how fast or slow you attempt to turn the driveshaft.

Gene
Gene Gillam

Guenter,
Assuming you have a Sierra 2.8 litre gearbox your mainshaft should have 7 teeth on it ( be aware that there are 6 and 8 tooth as well) and the drive gear should be 22 teeth.
There are a number of drive gears.
Yellow 22teeth, blue 23 teeth, white 25 teeth.
22 teeth can also be white which it is in my car.
www.miquip.co.za/.../The Ford Sierra service manual is online at this site and has all the info you will need.
Hope this helps
Tony
MGA MK2
A.D. Cox

Butch, do you sell conversions for the TD. I have a 4.1 Rear end in my car and obviously it is off with regards to the speedoo.
TOM Maine

...and another...

Butch... do you/could you make one for a stock speedometer running a 4.3 rear end.

I'm ready to pull the speedo and send it away...but its a bit chilly to roll the car down the drive and count revolutions...(I'm really surprised that by now the calibration companies haven't noted the ratios and just asked what speedo and what rear end??? I mean the way the speedo bounces... a bit one way or the other isn't going to matter....)
gblawson(gordon- TD27667)

After installing an MGB rear end, my approach to speedometer correction was to use GPS for accurate speed and then temporarily mark the glass. I was prepared to use CAD program to print out new dial face with numbers... but the cable proved frustratinjgly temperamental and now the speedo needle is totally uncooperative and erratic, so we're now used to looking at the tach for our speed.
JRN JIM

Guenter. and others. Good evening to you all.
It's 1900hrs now by me. I have a TD with
Hi gear conversion and a 5,125
ratio rear end. Re: speedograph richfield's form.
Jack up 1 one driving wheel turn the wheel
20turns and count the turns on the speedo cable. Mine was 14 7/8 turns. You can
calculate yours from these3 numbers.
Regards
Thoralf. Norway TD 4490.
btw I use GPS
Thoralf Sorensen (TD4490)

Thank you to all of you.
Obviously, nobody has the same Combination as I have. The Hi gear conversion using the FORD Sierra gear box and a MGA Rear axle with 4.3 Ratio.
Anyway, in the meantime I make a temporary repair of my Speedo drive and I checked the turns of the speedo cable as described by Thoralf in this thread.
The result is 11 3/4 per 20 wheel turns. Now I can calculate the ratio of the mini gearbox I will use to correct the reading with the reality.
I found in the Internet a figure for a Jaeger Odometer. The information was that the Speedo/Odometer combination as we have typically in our TD cars should need 1000 revolutions per mile. Who can confirm that?
Thank you all for your help,
Guenter
GK Guenter

I think the TD speedo is 1600 turns per mile. This is the number on the dial face. I have some Jaeger gauges with 1000 turns per mile but they are for a much later MG.
Cheers, Hugh
H.D. Pite

Guenter, the number should be on the face of your speedometer, e.g., 1600.

Bud Krueger

OK, thank you
1600 is given on the dial face.
Cheers,
Guenter
GK Guenter

Butch...

I have a 4.3 rear end... when I last checked on a measured highway, I turned .9 of a mile at the 10 mile marker...
Can you pm me

gblawson@vaxxine.com
gblawson(gordon- TD27667)

GUENTER. In my TD The speedo is JAEGER
S516 with the marking of 1600.
The speedo is not calibrated,so therefor I
use rhe GPS instead.
Thoralf. Norway TD 4490.
Thoralf Sorensen (TD4490)

Thank you all Gentlemen for your help your comments and your advices.
My way is clear now: In order to get a function able Odometer I will install a mini correction gearbox with a 21/13 Ratio. To protect the speedometer cable I will install the gearbox closed to the main gearbox.
With this is installed I can compare the Speedometer value with a GPS or with a bicycle Speedo or both. If I will find a heavy deviation the Speedo should readjusted by a professional hand.
Enclosed find the schematic from the TD drive gear with the different measurements I found on my car, the calculations I did and the result for the additional mini-gearbox. By the way, it will be a Bevel gear box with Modulus 0,5.
Any comment is appreciated.
Guenter


GK Guenter

Guenter,
Conrad Electronic sells a ready made 1:1 bevel stage with a 5mm input and output shaft with flats. Part no.222355-62
http://www.conrad.de/ce/de/product/222355/Modelcraft-Winkelgetriebe-11-Wellen-Durchmesser-5-mm?ref=searchDetail
I can send you my CAD drawing of it if you want. Go for a high number of teeth on the spur gears as it runs smoother which means you may have to increase the modulus.

Regards
Declan
Declan Burns

My way forward:
Enclosed please see my concept. I will make a small gearbox with a 1:1 bevel gear pair.
Next step then is a spur gear train with the 21/13 Ratio. All the gears are made from brass and they are commercially available: http://www.maedler.de/
The gear arrangement needs only very little space. I will design and build a compact aluminum housing for all elements with standard connection to the FORD gearbox and to the original MG Speedometer flexible cable.
By the way, the dimensions given in the drawing are Millimeter, not Inch.
Guenter

GK Guenter

This thread was discussed between 03/12/2013 and 10/12/2013

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